Lecture 4
More on membranes
glycosylation: adding a carbohydrate chain
can change cell properties
can attach to proteins, lipids
post-translationally (protein made first, then modified)
Fluidity of membranes
very important to the cell
If too rigid, some processes can’t happen (proteins can’t move)
If too fluid, membrane might fall apart, not sturdy enough
Has to be fluid enough to allow proteins to “float” around
Unsaturated:
has one or more double bonds, which causes a kink in the structure. This increases fluidity
Saturated:
every carbon is completely “saturated” with Hydrogen; NO double bonds. Makes a much straighter molecule. This decreases fluidity
Temperature and fluidity
warming increases fluidity: liquid crystal
cooling decreases fluidity: crystalline gel (firmer the liquid)
The transition temperature is the point that it goes from liquid crystal to crystalline gel.
In response to temperature change, the composition of the membrane can change in two ways:
desaturation of lipid
exchange of lipid chains
Some membranes have “ruffles” that allow cells to move and envelop other cells.
As humans, and the same with most other mammals, we have a pretty steady body temperature (37C). However, cold-blooded animals have enzymes that are used to change membrane fluidity to suit the temperature.
Eg. increase when colder by increasing unsaturated chains. Uses desaturase
Biological membranes are dynamic
Lipids are able to move easily laterally (within a layer/leaflet)
Lipid movement to other leaflets is much slower
There are some proteins that are not able to move
Membrane proteins can diffuse within bilayer, although it is restricted, long range is slow
Biochemical modification can dramatically alter protein mobility in the membrane
Cholesterol
regulates membrane fluidity
if added to liquid crystal: decreases fluidity
If added to crystalline gel increases fluidity
Is capable of embedding itself in lipid bilayer

Lipid Rafts
Small areas of plasma membrane that are less fluid than the surroundings
Some membrane proteins accumulate in rafts
relatively rigid
Function isn’t completely clear, could be a functional compartment
Movement across cell membranes
The lipid membrane acts as a barrier, does not allow everything through.
Uncharged, small particles can pass through (eg. Oxygen, Water**)
Large, charged particles cannot cross the bilayer easily
There are mechanisms that allow the transport of molecules/particles
**Water goes through aquapori
4 types:
Simple diffusion: Passive, non-mediated, Concentration outside the cell is higher than inside. Naturally goes from high concentrations to low concentrations. Very small, uncharged particles.
Diffusion through a channel: Passive, non-mediated. Think aquapori, substances go through a channel from high to low concentration. Small, charged particles (ions)
Ion channels are selective, and often times gated.
Voltage-gated: responds to change in charge
Ligand-gated: responds to certain molecule that binds with it
Mechano-gated: responds to physical force on membrane
Facilitated diffusion: Passive, mediated. Compound binds to facilitative transporter, an integral membrane.The transporter changes in conformation and allows the substance through to the other side.
Active Transport: Active, mediated. Compound binds to active transporter, which changes in conformation and allows to the other side. This goes against the concentration, so it goes from low concentrations to high concentrations.Requires input of Energy.

Well, looks like you get a portion of my lecture notes for cell biology.

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